1. Field of the Invention
The present invention relates to a control device and, in particular, the present invention relates to a servomechanism that controls a position and a speed of a load driven by a motor or the like.
2. Description of Related Art
There has been known a servomechanism that controls a position and a speed of a load which is an element driven by a motor or the like according to a target command, the servomechanism, for instance, being utilized for a numerical control machine tool or a numerical control coordinate measuring machine.
To drive and control a low rigid load causing vibrating response in a displacement, a speed and an acceleration thereof since a low rigid portion is provided at a connection with the motor for instance, a servomechanism has been utilized, which includes a quadruple loop control system having a load speed control loop in addition to a current control loop, a motor speed control loop and a position control loop (for instance, see Document 1: JP-A-2004-118635).
Such servomechanism is shown in FIG. 9.
In FIG. 9, a transfer characteristic of a motor speed control loop 910 with the current control loop is expressed as GM, and a characteristic of a load 110 is expressed as GF.
Though detail configuration is omitted, the current control loop has a motor, a motor drive power amplifier, a motor torque current detector and a current characteristic compensator, whereas the motor speed control loop has a motor rotation position detector detecting a rotation position of the motor, a differentiator calculating a rotation speed of the motor by differentiating the rotation position of the motor, and a motor speed characteristic compensator.
A load speed control loop 920 has a differentiator 921 calculating a load speed by differentiating a position of the load 110, and a speed characteristic compensator 922 provided with a proportional compensator (gain Kp) 923 and an integral compensator (gain Ki) 924.
A position control loop 930 has a position detector 931 detecting a position of the load 110 and a position compensator 932.
By having the load speed control loop 920, vibration suppressiveness of the control system can be enhanced even when the rigidity of the load 110 is low, thereby controlling the position and the speed of the load 110 stably and highly accurately.
However, since the load speed control loop 920 of which response is slower than that of the position control loop 930 is located inside the position control loop 930, the load speed may result in an overshoot depending on a setting value of the compensator (for instance, the gains of the proportional compensator 923 and the integral compensator 924).
For example, FIG. 11 shows a simulation result of a load speed when a ramped target position command pr as shown in FIG. 10 is input. Referring to FIG. 11, the load speed results in the overshoot in a transient state.